The present invention relates to a gene concerning S-adenosyl-L-methionine (hereinafter referred to as "SAM"). More particularly, the present invention relates to a gene enhancing an accumulation of SAM in a cell and a process for producing SAM using a cell having the gene.
It has been known that SAM participates in metabolism of fats, proteins, suger chains, and the like, and has effects on the therapies of excessive lipemia, arteriosclerosis, psychosis manifestations such as depression and neuropathic diseases, degenerative arthropathy neuropathic pain, sleeplessness, brain damage, and the like. Therefore, the mass production of SAM is desired.
SAM is a very unstable substance, and decomposes into 5'-methylthioadenosine and the like in a medium, though it is stable in a cell. Therefore, for the mass production of SAM, it is important how to keep SAM in a cell in a high concentration. It is well known that SAM can be accumulated in a high concentration in a cell when the cell is cultured in a medium containing methionine. For example, as to yeast, it is described in Journal of Biological Chemistry 229, 1037 (1957), Journal of Bacteriology 121, 267 (1975), Japanese Examined Patent Publication No. 17118/1977, and the like.
Further, for enhancing the accumulation of SAM to a higher concentration, there are processes in which other substances are added to the medium as well as methionine. There are exemplified, for instance, a process in which an amino acid not having sulfur atom is added (Japanese Unexamined Patent Publication No. 28296/1983), a process in which glycine or alanine is added (Japanese Unexamined Patent Publication No. 36397/1983), a process in which a heavy metal ion such as calcium ion or iron ion is added (Japanese Unexamined Patent Publication No. 40095/1983), a process in which a base is added (Japanese Unexamined Patent Publication No. 40096/1983), a process in which an ammonium salt or the like is added (Japanese Unexamined Patent Publication No. 138393/1983), a process in which a substance colouring in ninhydrin reaction is added (Japanese Unexamined Patent Publication No. 146274/1983), a process in which a specific amine is added (Japanese Unexamined Patent Publication No. 152497/1983), a process in which adenylic acid or the like is added (Japanese Unexamined Patent Publication No. 78594/1985), and the like. However, for practicing them industrially, there still remain problems that the content of SAM in a cell is not always increased that it is needed that cells are cultured in the medium containing expensive methionine in a high concentration, and the like.
As to several genera in bacteria, yeast, and the like, cells provided with ethionine-resistance by mutation treatment and the like have been already obtained. However, generally, they were not cells containing SAM in a high concentration, but cells in which the synthesis of SAM from methionine is suppressed because of a low activity of methionine adenosyltransferase (hereinafter referred to as "MAT") and the content of SAM is remarkably low (Mertz et al, Journal of Bacteriology 111, 778-783 (1972)). The above-mentioned cells show ethionine-resistance for the following reason. That is, the inhibition of growth of cells by a methionine derivative is caused through the steps, for instance in case of the methionine derivative being ethionine, that ethionine is converted into S-adenosyl-L-ethionine (hereinafter referred to as "SAE") by MAT, and that the SAE inhibit the methylation of SAM and also inhibit the growth of cells (for instance, Stekol, Advance in Enzymology 15, 369-393 (1963)). In ethionine-resistant cells, since the gene coding for MAT is destroyed by mutation treatment and therefore MAT activity is low, a synthesis of SAE from ethionine is inhibited (for instance, an example as to Escherichia coli, is described in Green et al, Biochemical and Biophysical Research Communication 38, 1120-1126 (1970)). Consequently, ethionine-resistance can be shown. Therefore, in the cells, the synthesis of SAM from methionine is also inhibited and a large amount of methionine is accumulated. In other words, ethionine-resistant cells are cells in which the accumulation of SAM is suppressed.